Dimercaptan terminated polythioether polymers and methods for making and using the same

ABSTRACT

Dimercaptan terminated polythioether polymers are disclosed, as are methods for making and using the same.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a divisional of U.S. patent application Ser.No. 11/260,553 filed Oct. 27, 2005, entitled: “DIMERCAPTAN TERMINATEDPOLYTHIOETHER POLYMERS AND METHODS FOR MAKING AND USING THE SAME”.

FIELD OF THE INVENTION

The present invention relates to dimercaptan terminated polythioetherpolymers. The invention is also directed to methods for making and usingthese polymers.

BACKGROUND OF THE INVENTION

Thiol-terminated sulfur-containing polymers are known to be well-suitedfor use in aerospace sealants due to their fuel resistant nature uponcrosslinking. Among the commercially available polymeric materials thathave sufficient sulfur content to exhibit this desirable property arethe polysulfide polymers described, for example, in U.S. Pat. No.2,466,963, and the alkyl side chain-containing polythioether polymersdescribed, for example, in U.S. Pat. No. 4,366,307. Materials useful inthis context also have the desirable property of liquidity at roomtemperature.

Another desirable combination of properties for aerospace sealants thatis much more difficult to obtain is the combination of long applicationtime or “pot life” (the time during which the sealant remains usable)and short curing time (the time required to reach a predeterminedstrength). High temperature resistance may also be desired for aerospacesealants. Compositions that have low Tg, are liquid at room temperature,have good pot life and/or good performance characteristics are desiredfor aerospace applications.

SUMMARY OF THE INVENTION

The present invention is directed to a dimercaptan terminatedpolythioether polymer having the formula (I):

wherein each R₁ is the same and is represented by the formula (I):

X is O or S; a is 2 to 6; b is 1 to 5; c is 2 to 10; n is 1 or greater;and R₂ is a diene residue other than a divinyl ether residue, a trieneresidue or a residue from an organic compound having a terminal leavinggroup.

Methods for making and using the polymer of formula (I) are also withinthe scope of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a dimercaptan terminatedpolythioether polymer having the formula (I):

wherein each R₁ is the same and is represented by the formula (II):

X is O or S; a is 2 to 6; b is 1 to 5; c is 2 to 10; n is 1 or greater;and R₂ is a diene residue other than a divinyl ether residue; a trieneresidue, or a residue from an organic compound having a terminal leavinggroup. A “diene residue”, “triene residue” and a “residue from anorganic compound having a terminal leaving group” will be understood asreferring to the moiety that remains upon reaction of the diene, trieneor compound with a leaving group, respectively. In certain embodiments,X is O and a, b, and c are all 2, n is 1, and R₂ is the residue ofvinylcyclohexene.

The polythioether polymers of the present invention are liquid at roomtemperature and pressure, have low Tg and have good fuel resistance. Asused herein, the term “room temperature and pressure” meansapproximately 77° F. (25° C.) and one atmosphere. “Low Tg” refers to aglass transition temperature of −50° C. or lower. In certain embodimentsof the present invention, the Tg is −55° C. or lower, such as −60° C. orlower or even −70° C. or lower. Low Tg is indicative of good lowtemperature flexibility in a cured formulation, which can be determinedby known methods, such as those described in Aerospace MaterialSpecification (AMS) 3267 §4.5.4.7, Aerospace Material Specification(AMS) 8802B § (AS 5127/17.6) and Military Specification (MIL-S)MIL-S-29574, and by methods similar to those described in ASTM D 522-88.Typically, good low temperature flexibility means no loss of adhesion tosubstrate, no cracking, no checking, etc. The polythioether polymers ofthe present invention may also exhibit very desirable fuel resistancecharacteristics when cured. One measure of the fuel resistance of apolymer is their percent volume swell after prolonged exposure to ahydrocarbon fuel, which can be quantitatively determined using methodsthe same as or similar to those described in ASTM D 792 or AMS 3269. Incertain embodiments, the present polythioether polymers, when cured,have a percent volume swell of 25 percent or less after immersion forone week at 140° F. (60° C.) and ambient pressure in jet reference fluid(JRF) type 1. In certain embodiments, the percent volume swell is 20percent or less.

The present polythioether polymers can have a number average molecularweight ranging from 200 to 8000, such as 200 to 4000, or 200 to 520.

It will be appreciated that the polythioether polymers of the presentinvention are polyfunctional; that is, they have two or more functionalgroups. At least two functional groups will be terminal mercaptangroups. Functionality can also be introduced via the R₁ and/or R₂moieties. For example, if R₂ is a triene residue, the functionality ofthe polythioether may be more than two. These polymers can be preparedby reacting two moles of the compound of formula (III):

HS—R₁—SH  (III)

where R₁ is as described above, with one mole of a diene, triene orother organic compound having a terminal leaving group. Any suitablediene, triene or other organic compound can be used includingcycloaliphatic, aliphatic and aromatic dienes and trienes. The diene isnot a divinylether. A particularly suitable diene is vinyl cyclohexene,such as 4-vinyl-1-cyclohexene, which will be understood as having theformula (IV):

It will be understood that when the diene is 4-vinyl-1-cyclohexene, thepresent polymer will have the structure (V):

Other suitable compounds include triallylcyanurate and di or tri halidessuch as dichloroalkane and trichloroalkene.

In certain embodiments, formula (III) is dimercaptodioxaoctane (“DMDO”).

Certain embodiments of the present invention, such as when formula (III)is DMDO and the diene is vinyl cyclohexene, result in a polythioetherpolymer that can be used to lower the viscosity of a composition; inthis manner, the compounds of the present invention can function asreactive diluents. This provides a significant advantage over the art;compositions utilizing the polymer of the present invention have a lowerviscosity without the use of solvent. Thus, solvent use can be minimizedif not eliminated according to the present invention. Elimination and/orminimization of solvent has obvious positive environmental implications;because there is little or no “flash off” of the solvent as itevaporates, handling of the present compositions is improved as comparedto other compositions having higher solvent content. In addition, theuse of the polymer of the present invention in a composition serves toslow down the cure of the composition, thus providing a longer “potlife” and/or time to apply the composition to a substrate. Accordingly,the present invention is further directed to a composition comprising apolythioether polymer as described above.

In addition to the polythioether polymer described above, thecompositions of the present invention can further comprise one or moreadditional sulfur-containing compounds, a curing agent and one or moreadditional additives. “Sulfur-containing compounds” and like terms referto any compound or blend of compounds wherein at least one componentcomprises a sulfur molecule. Examples can include those known in theart, such as polysulfides, and/or polythioethers, such as thosepolysulfides commercially available from Akzo Nobel under the nameTHIOPLAST and from Toray under the name THIOKOL LP, and such as thosepolythioethers described in U.S. Pat. Nos. 6,172,179; 5,912,319; and4,609,762. In certain embodiments, the sulfur-containing compoundscomprise a polymer blend comprising a polysulfide component and apolythioether component, such as that described in U.S. application Ser.No. 10/935,857, hereby incorporated by reference. It will be appreciatedthat the present polymers are themselves sulfur-containing compounds andcan be used in the present compositions without any othersulfur-containing compounds. The term “curing agent” refers to amaterial that can be added to the present composition to accelerate thecuring or gelling of the sulfur-containing compound(s). “Curing”, “cure”and like terms refer to the point at which the composition achieves acure hardness of 30 Durometer “A” as measured according to ASTM D2240.Any suitable curing agent can be used. In certain embodiments, thecuring agent contains oxidizing agents that oxidize terminal mercaptangroups to form disulfide bonds. Suitable curing agents include, forexample, lead dioxide, manganese dioxide, calcium dioxide, sodiumperborate monohydrate, calcium peroxide, zinc peroxide, dichromate andepoxy.

Various additives such as fillers, adhesion promoters and plasticizerscan also be used in the present compositions. Fillers useful in thepresent compositions, especially for aerospace applications, includethose commonly used in the art, such as carbon black, calcium carbonate(CaCO₃), silica, nylon, microspheres and the like. In one embodiment,the compositions include 10 to 70 weight percent of the selected filleror combination of fillers, such as 10 to 50 weight percent based uponthe total weight of the composition.

One or more adhesion promoters can also be used. Suitable adhesionpromoters include phenolics such as METHYLON phenolic resin availablefrom Occidental Chemicals, and/or organosilanes such as epoxy, mercaptoor amino functional silanes such as A-187 and A-1100 available from OSiSpecialties. In one embodiment, an adhesion promoter is used in anamount from 0.1 to 15 weight percent based upon total weight of theformulation.

A plasticizer can be used in the compositions, often in an amountranging from 1 to 8 weight percent based upon total weight of theformulation. Useful plasticizers include phthalate esters, chlorinatedparaffins, and hydrogenated terphenyls.

The compositions can further comprise one or more organic solvents, suchas ethyl acetate, often in an amount ranging from 0 to 15 percent byweight on a basis of total weight of the formulation, such as less than15 weight percent or less than 10 weight percent.

Compositions of the present invention can also optionally include otherstandard additives, such as pigments; thixotropes; retardants;catalysts; and masking agents.

Useful pigments include those conventional in the art, such as carbonblack and metal oxides. Pigments can be present in an amount from 0.1 to10 weight percent based upon total weight of the composition.

Thixotropes, for example fumed silica or carbon black, can be used in anamount from 0.1 to 5 weight percent based upon total weight of thecomposition.

Certain compositions of the present invention comprise 1 to 30 weightpercent, such as 5 to 20 weight percent of the mercaptan terminatedpolythioether polymers of the present invention; 20 to 70 weightpercent, such as 35 to 50 weight percent, of a sulfur-containingcompound other than the present polymer; from 1 to 50 weight percent,such as 5 to 25 weight percent or 10 weight percent of a curing agent;and 10 to 50 weight percent, such as 5 to 30 weight percent or 25 weightpercent of other additives, with weight percent being based on the totalweight of the composition. For compositions in which the present polymeris the only sulfur-containing compound, these weight percentages areadjusted accordingly.

The present compositions are typically packaged as a two-component or“2K” system. One component comprises the polythioether polymer of thepresent invention, and optionally other sulfur-containing compound(s)and may also contain one or more of various additives, such asfiller(s), extender(s), adhesion promoter(s), accelerator(s), and/orretardant(s); the second component generally comprises the curing agentand may also contain one or more of various additives, such asplasticizer(s), filler(s), accelerator(s), and/or retardant(s). The twocomponents are mixed immediately prior to use. For example, the twocomponents can be mixed using a specially adapted cartridge/rod systemsuch as that commercially available from PRC-DeSoto International, Inc.as SEMKIT. Alternatively, the components can be mixed together and themixture maintained at a temperature below that at which the curing agentis reactive. The term “reactive” means capable of chemical reaction andincludes any level of reaction from partial to complete reaction of areactant. A storage temperature of below −40° C. is typically suitableto retard or prevent the reaction between the curing agent and thepolythioether polymer component.

A particularly suitable use for the compositions of the presentinvention is in all classes of sealants. A sealant according to thepresent invention is one that comprises any of the mercaptan terminatedpolythioether polymers described herein. “Sealant” and like terms referto compositions that have the ability to resist atmospheric conditionssuch as moisture and/or temperature and/or at least partially blocktransmission of materials such as water, fuel, and/or other liquids andgases. Sealants often have adhesive properties, as well. Sealants aregenerally identified by “Class”, based on their viscosity. Class Asealants generally have a viscosity of 100 to 400 poise. Because thesesealants are typically brush-applied, they are often referred to as a“brush coat”. Class B sealants typically have a viscosity of 6000 to18000 poise, and are typically applied through use of an extrusion gunor a spatula. These sealants may be used on a fillet seal, and arecommonly used as fuel tank sealants. Class C sealants generally have aviscosity between that of Class A and Class B sealants, such as from1000 to 4000 poise. Class C sealants can be applied in various manners,such as with a brush, roller or extrusion gun, and may be used for a“fay seal”. Accordingly, the present invention is further directed to asealant comprising any of the compositions comprised above.

The compositions of the present invention can be applied to any numberof substrates including, for example, titanium, stainless steel,aluminum, and the primed, organic coated and chromate forms thereof,epoxy, urethane, graphite, fiber glass composite, KEVLAR, acrylics andpolycarbonates. As noted above, the present compositions areparticularly useful in aerospace applications, such as aerospacesealants and the linings for fuel tanks, fuselages, and the like. Anaerospace sealant material according to the present invention canexhibit properties including high temperature resistance, fuelresistance and/or flexural strength. The formulations described hereinmay also be well suited for uses in which temperature extremes,chemically harsh environments and/or mechanical vibrations areexperienced. The present formulations may be used outside of aerospaceapplications as well.

The polymer of the present invention is generally ungelled, which meansthat the polymer blend has an intrinsic viscosity that can be measured.The cured formulations of the present invention generally have good lowtemperature flexibility, which is desired in aerospace applicationssince the formulations are subjected to wide variations in environmentalconditions, such as temperature and pressure, and physical conditionssuch as joint contraction and expansion and/or vibration.

Viscosities as reported herein are measured at a temperature of about25° C. and a pressure of about 760 mm Hg determined according to ASTMD-2849, paragraphs 79 to 90 using a Brookfield Viscometer.

The present invention is further directed to methods for sealing anaperture comprising applying to a surface associated with the apertureany of the sealants described herein. An adhesion promoter can beapplied to the surface prior to the sealant if desired. “Aperture”refers to a hole, gap, slit or other opening. In certain embodiments,the aperture is on an aviation or aerospace vehicle.

As used herein, unless otherwise expressly specified, all numbers suchas those expressing values, ranges, amounts or percentages may be readas if prefaced by the word “about”, even if the term does not expresslyappear. Any numerical range recited herein is intended to include allsub-ranges subsumed therein. Plural encompasses singular and vice versa.For example, while reference is made herein to “a” polythioetherpolymer, one or more polythioether polymers, at least one of which iswithin the present invention, can be used. Similarly, one or moresulfur-containing compounds, one or more curing agents and/or one ormore of any of various additives can be used in the presentcompositions. Also, as used herein, the term “polymer” is meant to referto prepolymers, oligomers and both homopolymers and copolymers; theprefix “poly” refers to two or more.

EXAMPLES

The following examples are intended to illustrate the invention, andshould not be construed as limiting the invention in any way.

Example 1

1,8-Dimercapto-3,6-dioxaoctane (616.95 g, 3.38 moles CAS #14970-87-7)was charged into a 1 liter four-necked round bottom flask equipped witha mechanical stirrer, thermometer, and two gas passing adapters (one forinlet of nitrogen and the other for outlet). The flask was flushed withdry nitrogen and the contents were heated to 32° C.

4-Vinylcyclohexene (183.05 g, 1.69 moles, CAS #100-40-3) was added intostirring dithiol over 3 hours, 15 minutes. Mild exotherm developedduring the addition but the temperature was maintained at 42° C. to 45°C. The reaction mixture was heated for 3 hours at 55° C. Five portions(140 g each) of free radical initiator (VAZO-52 (2,2′azobis)2,4-dimethylpentanenitrile, CAS #4419-11-8, commercially available fromDuPont) were added over an interval of 2 hours at 55 to 60° C. Thereaction mixture was evacuated at 70 to 75° C./7 to 8 mmHg for 2 hoursto give a clear, colorless product. Yield: 800 grams (100%); mercaptanequivalent: 238 (theoretical: 236); viscosity: 1.331 poise.

Example 2

A Class B sealant was prepared by mixing the components listed in Table1 in a double shaft cowles type batch mixer until homogeneous under avacuum of 27 inches of mercury or greater. Components A and B were mixedin a weight ratio of 100:12 to prepare the final sealant.

TABLE 1 Component A Weight Percent Ingredient Sample 1 Sample 2 Sample 3Sample 4 Ethyl Acetate 1 1 1 1 VCH extended DMDO 7 6 7 7 preparedaccording to Example 1 Modified polysulfide¹ 6.5 — 6.5 — Modifiedpolysulfide² — 6.5 — 6.5 LP-55³ 46 46 45 45 Polythioether⁴ 12.5 12 12.512.5 Sulfur-containing 2.5 2.5 2.5 2.5 phenolic Sulfur-containing 1.751.75 1.7 1.7 phenolic Polysulfide latex 1 1.3 1.4 1.4 Sulfur 0.1 0.20.17 0.17 Silica 6.5 5.5 5.5 5.5 Precipitated CaCO₃ 10 10 10 10 Nylon⁵5.5 6 5.5 5.5 Polymeric microsphere⁶ 1.06 0.95 0.93 0.93 Mercaptosilane⁷ 0.2 0.2 0.2 0.2 Amino silane⁸ 0.12 0.1 0.1 0.1 Viscosity (poise)9600 8400 6600 6800 Component B Ingredient Weight Percent MnO₂ 55Plasticizer⁹ 38 Stearic acid 0.6 Sodium stearate 0.7 Molecular sievepowder¹⁰ 0.7 Dipentamethylene/thiuram/ 5.0 polysulfide mixture¹¹¹Prepared according to U.S. Pat. No. 4,623,711 (molecular weightapproximately 2200). ²Prepared according to U.S. Pat. No. 4,623,711(molecular weight approximately 2500). ³THIOKOL LP-55 polysulfidepolymer, commercially available from Toray Fine Chemicals. ⁴Preparedaccording to U.S. Pat. No. 6,172,179 (molecular weight approximately3000. ⁵ORGASOL, commercially available from Atofina. ⁶EXPANCEL,commercially available from Akzo. ⁷Commercially available from OSi.⁸Commercially available from OSi. ⁹Partially hydrogenated terphenyl(HB-40), commercially available from Solutia. ¹⁰Alkali metalalumino-silicate 3-angstrom powder, commercially available from UOP.¹¹Akrochem Accelerator DPTT Powder, commercially available from AkrochemCorp.

The sealant of Sample 2 was tested to the requirements of proposedspecification BMS 5-142 with excellent results, as illustrated in Table2 below.

TABLE 2 Test Property Requirement Sample 2 Viscosity (poise) 6,000 to13,000 8,400 Specific gravity (max) 1 to 10 1.04 Nonvolatile content, %min 90 to 0  96 Flow (inch) 0.10 to 0.50  0 to 10 Application time after1 hour, g/min 15 48 (min) Tack-free time, hours (max) 12 <2 Cure rateafter 24 hours (min) 30 34 @ 6 hours Weight loss % (max) 16.0 7.26Flexibility No cracking, Passed checking or delamination Dry tensilestrength psi (min) 150 210 Type III* Immersion tensile strength 150 225psi (min) Dry elongation % (min) 150 220 Type III Immersion elongation %(max) 150 220 Moisture and fuel stability % (max) distilled water, 5days @ 120° F. 20.0 19.3 Type III*, 5 days @ 120° F. 5.0 2.34 *Jetreference fluid TT-S-735A Type III

Sample 2 also had good peel strength and showed resistance tohydrocarbon fluid when tested according to the same specification.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

1. A composition comprising: a) dimercaptan terminated polythioetherpolymer having the formula (I):

wherein each R₁ is the same and is represented by the formula (I):

X is O or S; a is 2 to 6; b is 1 to 5; c is 2 to 10; n is 1 or greater,and R₂ is a diene residue other than a divinylether residue, a trieneresidue, or a residue from an organic compound having a terminal leavinggroup; and b) a polymer blend comprising a polysulfide and apolythioether.
 2. The composition of claim 1, wherein X is O; a is 2; bis 2; c is 2; n is 1, and R₂ is the residue of vinyl cyclohexene.